feat: implement user registration functionality

- Created registerUser server action with email/password validation
- Implemented password hashing with bcrypt (10 rounds)
- Added email uniqueness check to prevent duplicate registrations
- Created session after successful registration using NextAuth
- Added comprehensive validation for email format and password length (min 8 chars)
- Added email and password validation utilities to lib/validation.ts
- Removed PrismaAdapter from auth.ts (using JWT sessions only)
- Added vitest testing framework and configuration
- Created comprehensive unit tests for registration functionality
- All tests passing (6/6)

Requirements covered: 1.1, 1.2, 1.3, 1.4, 1.5

auth.ts

@@ -1,6 +1,5 @@
 import NextAuth from "next-auth"
 import Credentials from "next-auth/providers/credentials"
-import { PrismaAdapter } from "@auth/prisma-adapter"
 import { prisma } from "@/lib/db"
 import bcrypt from "bcryptjs"
 import { z } from "zod"
@@ -12,7 +11,6 @@ const loginSchema = z.object({
 })
 
 export const { handlers, signIn, signOut, auth } = NextAuth({
-  adapter: PrismaAdapter(prisma),
   session: {
     strategy: "jwt",
   },